Presentation on theme: "Chapter Thirteen Mass Movement. Mass Movement Process that transports Earth’s materials downslope by the pull of gravity Friction, strength, and cohesiveness."— Presentation transcript:
Chapter Thirteen Mass Movement
Mass Movement Process that transports Earth’s materials downslope by the pull of gravity Friction, strength, and cohesiveness of materials resist mass movement Angle of slope (sloppiness), water content, lack of vegetation, and biological disturbances enhance mass wasting Extent of damage and deaths from Mass Movement in the USA (1925-1975): 1 k/20 k (75 b$ as compared to 20 b$ for all others)
What causes Mass Movement Principal Factor: Gravity Principal Resistance Factors: Friction, strength and cohesiveness of slope material Gravity Components: – Parallel to Slope (Gd) – Perpendicular to Slope (Gp) Amount of Friction & Steeper slopes depend on: Via Natural processes (Faulting, Folding and Tilting of strata), River cutting, Glacial Erosion & Coastal wave cutting Via Human causes: Quarrying, Road cutting and Waste dumping
Boulder on a hillside
Slope Composition Factors that can reduce rock stability are: – Breakage into networks of joints, fractures, or faults – Mechanical weathering processes – Sedimentary bedding planes – Cavities formed in soluble rock via dissolution – Igneous cooling joints (A joint pattern developed during cooling) – Foliated metamorphic rock with marked rock cleavage – Weakness planes parallel to slope
Causes of Mass Movement Steepness of Slope – Stable only if friction is greater than G d Faulting, folding, river cut, glacial, coastal wave create steep slope; angle of repose (angle at which an unconsolidated material is stable; 30-35° for dry sand); Different for different material; determined by particle size, shape and particle arrangement; for talus slopes >40 ° Composition of Material either promotes or resists mass wasting Solid /Unconsolidated Vegetation- lack of which promotes mass wasting (Binds and stablizes loose, unconsolidated material) Water Content- increases weight of material and reduces friction between planes of weakness; small water increases cohesiveness Human/Other Disturbances
Causes of mass movements – contd. Excess of water reduces friction between surface materials and underlying rocks Reduces cohesiveness – counteract some or all of G p by buoying upward the weight of slope material Triggers for Mass Movement Events Natural Triggers Climatic- torrential rains and snow melt Geologic- earthquakes and volcanic eruptions Human-Induced Triggers Oversteeping of slopes- excavation
Causes of mass movements – contd. Overloading- excess water, building, and other construction Deforestation/overgrazing of vegetation Loud noise- trains, aircrafts, blasting Lawn sprinkling
Processes that oversteepen slopes – contd.
Slopes susceptible to mass movements
Particle size and shape
Effect of particle arrangement on stability
Water can cause failure in slopes of solid bedrock
Effect of water – contd.
Saturation with water promotes mass movement
Turtle Mountain landslide
Types of Mass Movement Classification based on velocity & Composition Slow Mass Movement Creep: Slowest Mass movement (measured in mm or cm per year); affects unconsolidated materials – Causes: Burrowing animals Trampling of animals Splashing of raindrops Swaying of plants and trees Freezing and Thawing Wetting and Drying of clays
Types of Mass Movements – contd. Solifluction: Special variety of creep, soil flow – Comparatively fast form of creep – Occurs in permafrost areas – Indications of creep-tilted ‘vertical’ structures Rapid Mass Movement: – Measured in km/hr or m/s; classified by types of motion; one type can evolve into another – Falls: Fastest type of rapid mass movement; rocks break free steep slope & plummet to the ground
Rapid Mass Movement
Rapid Mass Movement – contd. Slides and Slumps: – Slides: Single intact mass of rock. Soil, or unconsolidated material moves along a preexisting plane of weakness Unconsolidated sediment: Between material types is slip plane Solid Bedrock: within the rock, along planes of weakness Sedimentary rock: Bedding planes Plutonic igneous rocks: Large joint produced by exfoliation Metamorphic rocks: Along foliation surfaces
Rapid Mass Movement – contd. Slumps: Slides with concave slip planes – forms scarps (Steep, exposed cliff face that forms where the slump mass pulls away) Flows: – Mixture of solid, unconsolidated particles that moves downslope like a viscous fluid – Earthflows: Relatively dry masses of clayey or silty regolith (~ meter/hour) – Mudflows: Swifter-flowing slurry of regolith mixed with water; likely to develop after heavy rainfalls – Quick Clays: Highly fluid mudflow – Lahars: Catatrosphic mudflows (volcanic eruption: St. Helens) – Debris Flows: Triggered by the sudden introduction of large amounts of water; coarser than sand, often contain boulders – Debris Avalanches: Swiftest & Most dangerous mud flows
The development of quick-clay flows
The development of quick-clay flows-contd.
Rapid Mass Movement map
Dating Mass Movement events
Avoiding/preventing Mass Movement Avoiding Predicting mass movement Terrain analysis (composition, layering, structure, water content and drainage), field visit, eye witness/recorded accounts Vegetation- over grazing, harvesting Preventing Develop Prevention Plan Enhance Forces that Resist or Reduce forces of mass wasting Structural Approach- reduce slope (Modification: unloading, grading & removing material), Bolts and Pins Non-Structural Approach- tree, chemical stability
Slide Prevention – contd.
Structural supports to prevent slope failure
Extraterrestrial Mass Movement Mass movement on the Moon – by dry process – Triggered by Meteorites (which produces avalanches) Mass movement on Mars: Triggered by Meteorite impacts Slides, slumps, and debris and mud flows reported